The present invention relates to the field of distance detectors and proximity sensors.
Conventionally, a proximity detector comprises an oscillating circuit with a coil and capacitor, the Q factor of which depends upon the distance between the coil and a movable metal part constituted by the object whose distance to the coil is to be detected or by a metal part associated with the movement of this object. Various devices are provided for measuring the variation of the Q factor, by detecting the amplitude variation of the oscillations or by causing those oscillations to stop when a critical distance is reached.
As explained in detail in the European Patent Application EP-A-0 070 796 in the name of company Siemens AG, those proximity detectors are very sensitive to temperature variations, which substantially reduces their detection capability if no compensation step is taken.
Thus, this document EP-A-0 070 796 discloses a process for permitting the oscillation amplitude of the resonant circuit to remain constant whatever the temperature variation may be.
This process is described in relation to FIG. 1 and consists of using an a.c. current source I.sub.B at a resonance frequency and constant amplitude. Current flows from terminal B to terminal A of a resonant circuit constituted by a winding L and a capacitor C and through an inductor L, the second terminal of which is connected to a point D. Windings L and L' are in fact constituted by separate windings of the same coil divided and isolated one from the other. Thus, when a current flows between terminals B and D passing through windings L and L', the latter being considered as a pure resistance, the inductive effects are compensated for, due to the high coupling between the windings. When a temperature variation occurs, the resistance of winding L increases, the Q factor of the oscillating circuit therefore decreases, that is, the amplitude of the oscillations decreases. But simultaneously, the voltage across L, that is, across the oscillating circuit, increases since it is equal to the resistance of L multiplied by current from I.sub.B and this resistance has increased. Consequently, the decrease of the Q factor is compensated for by the increase of the voltage across the terminals and the oscillation amplitude of the resonant circuit remains substantially constant even though the temperature varies. Therefore, a resonant circuit supplying oscillations having substantially constant amplitude independent of temperature variations has been achieved.
However, a problem is encountered when it is desired to use such a circuit as a proximity detector or as a distance detector. Indeed, the resistance that has been hitherto considered as being the resistance of winding L is in fact the equivalent parallel resistance of the resonant circuit at the oscillation frequency. This equivalent resistance depends upon the coupling factor of the coil with a metal part, the displacement of which is to be detected. Thus, the above described circuit tends to supply oscillations of constant amplitude not only when the temperature varies but also when the aforementioned coupling factor varies. Therefore, this circuit will exhibit little sensitivity as a proximity detector since it tends to compensate for the phenomenon it is desired to detect. In any case, it does not permit an indication on distance variations between a metal part and the coil.
Thus, an object of the present invention is to provide a distance detector, usable as a proximity detector, independent of temperature variation effects.